1. Field of the Invention
This invention relates to a small-sized electric motor used for actuating a seat, a steering wheel, or the like in automotive vehicles, for example, and more particularly, to a casing structure of the small-sized electric motor.
2. Description of the Prior Art
Heretofore, there has been used small-sized electric motors as shown in FIGS. 9A, 9B and FIG. 10 for the aforementioned purpose.
A small-sized electric motor 100 shown in FIGS. 9A and 9B is provided with a yoke 102 formed with a hollow cylinder having an oval shaped cross section, an end cap 103 secured on the left-side end of the yoke 101 and a front cover 105 secured on the right-side end of the yoke 102 as shown in FIG. 9A. The end cap 103 and the front cover 105 are provided with bearings 103a and 105a, respectively. An armature 106 is housed in the yoke 102 and supported rotatably by the bearings 103a and 105a.
The yoke 102 is formed by rolling a rectangular shaped metallic plate 101 into an oval shape. The metallic plate 101 is provided with jagged faces 101c and 101d on both ends 101a and 101b thereof respectively, which are engaged with each other at the time of forming the metallic plate 101 into the oval shape, and the jagged faces 101c and 101d are composed of curved parts 101e, 101f and straight parts 101g, 101h which are parallel to the axis a of the yoke 102 and connect the curved parts 101e and 101f. The yoke 102 is formed so that the jagged faces 101c and 101d may be situated in the center b between an upper face 102a and a bottom part 102b of the yoke 102.
At the time of rolling the metallic plate 101 into the oval shape, the curved parts 101e and 101f are contacted closely by compressing the metallic plate 101 in the axial direction after engaging the curved (projecting) parts 101e on the jagged face 101c into the curved (concaved) parts 101f on the jagged face 101d and contacting the straight parts 101g and 101h with each other on the jagged faces 101c and 101d of the metallic plate 101.
However, in the small-sized electric motor 100 as mentioned above, a side plate 102c of the yoke 102 which is a part from the upper face 102a to the jagged face 101c of the metallic plate 101 and a side plate 102d of the yoke 102 which is a part from the bottom part 102b to the jagged face 101d of the metallic plate 101 sometimes vibrate sympathetically according to the rotation of the armature 106, and there is a problem since noise from the motor 100 becomes larger by the sympathetic vibration.
Additionally, there is another problem in that there is the possibility that the straight parts 101g and 101h are deformed to cause gaps therebetween because the yoke 102 of the motor 100 is formed by compressing the rolled metallic plate 101 in the axial direction at the state in which the curved parts 101e and 101f are engaged with each other and the straight parts 101g and 101h are in contact with each other as mentioned above, and the compressive force is applied in parallel to the straight parts 101g and 101h.
On the other hand, there has been also used a small-sized electric motor shown in FIG. 10.
A small-sized electric motor 110 shown in FIG. 10 is provided with an armature 111, a yoke 112 housing the armature 111, a gear casing 114 rotatably supporting an output shaft 113 and a conical shaped end cap 115 secured to the yoke 112 on the right side in the figure.
The yoke 112 is formed with a bearing fitting part 112a by two-step drawing on the right side thereof, an armature shaft 111a of the armature 111 is supported rotatably by a bearing 116 disposed on the bearing fitting part 112a and bearings 117, 118 disposed on the both sides of the gear casing 114 as shown in FIG. 10.
The end cap 115 is secured to the right side of the yoke 112 to cover the bearing fitting part 112a by spot welding at points 115a.
The yoke 112 is provided with a tab 112b formed by bending perpendicularly a tongue protruding from a contacting face 112c on the left side of the yoke 112, and the tab 112b is provided with a through hole for connecting the gear casing 114 therewith.
The gear casing 114 is housed with a worm 111b formed on the armature shaft 111a of the armature 111 and, a worm wheel 113a formed on the output shaft 113 and meshed with the worm 111b of the armature shaft 111a. The gear casing 114 is fixed on the left side of the yoke 112 at the contacting face 112c by tightening a screw 119 passing through the through hole of the tab 112b into a fitting part 114a of the gear casing 114.
However, in the small-sized electric motor 100 shown in FIG. 10, the bearing fitting part 112a is formed by two-step drawing as described above. There is the possibility that the wall thickness of the bearing fitting part 112a is not uniform because the bearing fitting part 112a is formed by the final step of the drawing. In such a case, the bearing 116 is easy to deviate from the center line of the motor 110, and there is a problem in that it is difficult to incorporate the armature 111 into the yoke 112 accurately and it is impossible to obtain desired rotational accuracy.
In addition to above, concerning the connection between the yoke 112 and the gear casing 114 through the tab 112b, the tab 112b is formed by bending the tongue provided on the side of the yoke 112 perpendicularly, therefore it is difficult to form the tab 112b so as to coincide the connective face 112d of the tab 112b with the contacting face 112c of the yoke 112. Accordingly, there is another problem since it is difficult to connect the gear casing 114 with the yoke 112 accurately and it is also impossible to obtain desired rotational accuracy in case there is a level gap between the connective face 112d of the tab 112b and the contacting face 112c of the yoke 112 in addition to a problem of the airtightness.